1. Field of the Invention
The invention generally relates to an image noise reduction method, and more particularly, to a method of multi-frame image noise reduction.
2. Description of Related Art
Usually, image noise can be primarily divided into impulse noise and Gaussian noise. Gaussian noise is usually processed with finite impulse response method (FIR method), i.e., convolution-based method. In terms of processing impulse noise, median filter method is a more effective way. However, the most problematic issue of the median filter method rests in it easily destroys integrity of an edge and also easily makes the edge look indistinct.
The conventional median filter method is applied in three-dimensional noise reduction (3D NR) and has roughly following two schemes. One of the schemes is to set a window with a certain size, followed by performing median filter operation on the pixels in each window of all temporal frames. Another one is to set a window with a certain size, followed by weighting current pixels and then, performing median filter operation on the pixels in each window of all temporal frames. The above-mentioned schemes that performing median filter operation on the pixels in each window of all temporal frames without any judgement on edges would destroy the detail of an edge, and at the boundary between a smooth region and an edge, it possibly occur that the detail of an edge replaces a part of a smooth region; that is, a weird situation may occur that the edge pixels present themselves on a smooth region. Although the above-mentioned second scheme increases the weights of the current pixels, but under the situation without judgement on edge direction and the presence of edge, the risks of destroying the detail and wrongly replacing a part of a smooth region with the edge pixels are still unavoidable.